The invention relates to an electronically commutated motor comprising P phases and N pairs of poles.
On account of their high specific power, electronically commutated motors, or brushless motors, can be used to control the rotation of a mechanical member in a large number of applications.
One particular example of such an application relates to electric power-assisted steering systems in which an electronically commutated motor can be integrated so as to allow the application of an assist torque for the angular displacement of the wheels of a vehicle.
In a variant, and in the case of a xe2x80x9csteer by wirexe2x80x9d type decoupled steering system in which there is no direct mechanical link between the steering wheel and the wheels of the vehicle but only an electrical connection, an electronically commutated motor can be disposed under the steering wheel so as to reconstruct the steering force at the steering wheel. In a variant, a motor can be integrated, possibly in addition to the one mentioned above, so as to provide the angular displacement of the wheels of the vehicle.
Within the scope of its application, it is necessary to control precisely the commutation of the current in the phases of these motors in order to obtain a torque which is optimum and equal to a set point with the fewest modulations possible.
To do this, use is known of a sensor comprising three sensing elements disposed opposite an encoder having N pairs of poles, said encoder being rotated by the motor.
In the case of a three-phase motor, by answering that the three sensing elements are phase-displaced with respect to one another by a mechanical angle making it possible to supply three electrical signals phase-displaced by 120 electrical degrees, it is possible to control the commutation between the phases of the motor at the right moment.
But this type of implementation can cause modulations of the motor torque which are due in particular to:
the reluctance torque of the motor;
faults of commutation of the current from one phase to another depending on the speed of rotation of the motor;
the absence of overlapping of the trapezoidal EMFs.
There is known, in particular from the document FR-2 749 452, a device for controlling an electronically commutated electric motor which makes it possible to partially avoid the torque faults related to the commutation of the current from one phase to another.
But, in order to avoid the torque modulation related to the motor design (reluctance torque and absence of overlapping of the trapezoidal EMFs), it then becomes necessary to know to a fine degree the absolute position of the rotor in order to control the current in the phases according to the position of the rotor.
Such a control strategy therefore necessitates an additional sensor giving a high-resolution position in addition to the one controlling the commutation.
This type of solution, providing two sensors, leads to significant integration constraints and a non-optimised cost. Moreover, it can be penalising in terms of mechanical inertia of the rotor.
The invention aims in particular to resolve these drawbacks by proposing an electronically commutated motor in which one of the anti-friction bearings allowing the rotation of the rotor carries a two-track encoder which is movable rotation-wise in front of a single sensor fixed rotation-wise, said sensor being capable, after processing by a control device, of delivering on the one hand signals for commutating the current in the phases and on the other hand high-resolution position signals for allowing the control of the current supplying the phases, with a sufficiently fine resolution to avoid motor torque modulations.
Moreover, the motor according to the invention requires only a small amount of mechanical modification compared with conventional motors, while integrating a device for controlling the commutation of the current between the phases of the motor which is effective.
To that end, the invention proposes an electronically commutated motor comprising P phases and N pairs of poles, said motor comprising:
a rotor having the N pairs of poles and a stator having P coils formed respectively from a phase winding, said rotor being mounted able to rotate inside the stator by means of at least one anti-friction bearing, said anti-friction bearing comprising a fixed ring, a rotating ring associated with the rotor and rolling bodies disposed between said rings; and
a device for controlling the commutation of the current between the phases of the motor;
said motor also comprising:
a circular multipole encoder associated with the rotating ring of an anti-friction bearing, said encoder comprising a main multipole track and a multipole commutation track comprising 2*P*N singularities equidistributed angularly, the main and commutation tracks being concentric on the encoder; and
a control device comprising:
a fixed sensor disposed opposite and at air gap distance from the encoder, said sensor comprising at least three sensing elements, at least two of which are positioned opposite the main track so as to deliver square digital signals A, B in quadrature, and at least one of which is positioned opposite the commutation track so as to deliver a signal C in the form of 2*P*N pulses per revolution of the encoder;
a circuit for commutating the currents in the phase windings of the motor having 2*P*N switches;
a control circuit for the commutation circuit which:
from the signal C supplies commutation signals for the switches; and
from the signals A, B and according to a first current set point defines a second current set point which controls the current supplying the phase windings of the motor.